Do you know what fins, barrels, epidermis, and gills are and what their functions are? Do you know that the cuticle layer is considered the first layer of defense against disease and parasites? Learning a Koi's anatomy and their functions could help you better care for your Koi through observation and behavior. 

External Anatomy

Skin - The top layer of skin is the epidermis which is covered with a layer of mucus which is a barrier to disease and parasites as well as aides is movement by reducing friction.

Scales - Scales are thin, flexible plates. Scales are located under the epidermis in the layer called the dermis. They are attached at the forefront and free at the backend  and overlap. Scales are an additional layer of protection, sometimes referred to as armor. Some Koi develop fukurin. Fukurin occurs when the dermis develops under the scales and has a very distinct net appearance. The age of a Koi can be determined by counting the rings on the scale.

Fins - Each fin has a specific purpose. The caudal or tail fin is for forward swimming and quick reaction. The dorsal or top fin is for stabilization when forward swimming. The anal fin is also used for stabilization. The pectoral fins are paired and have numerous functions, like steering, used for slow swmimming forward and backward, braking and to counteract the jet effect from the water being forced out the operculum openings. They are also useful to stir up sediment on the bottom of the pond to help look for food. The pelvic or ventral fins are also a pair and control pitch and roll and ascending and descending. Because fns are easy to see, they are also good indicators of problems, like disease, parasites, hemorrhaging, fraying, torn/damaged. 

Barbels - There are a set of Barbels. They have taste buds at the ends and help determine whether something is tasty or not. Barbels are helpful when foraging pond bottoms to locate acceptable food.

Operculum - This is the covering over the gills which protects them. Its primary function is to control the water pressure as it passes over the gills. It also acts as a valve to let the deoxygenated blood out while stopping it from coming back in. It is not attached at the back and lower portion allowing careful inspection of the gills and must be handled with care so it is not torn or damaged. 

Mouth - Koi have a protrusive mouth for bottom feeding, however can feed at any level.

Hearing - Koi do not have external ears, but hear through an internal ear known as the Weberian ossicle through bones connected to the cranial swim bladder. They are very sensitive to noise and can be stressed, become ill and have even died when exposed to loud noise, example banging on ice on a frozen pond.

Eyes - Koi have excellent vision with an eye on each side of the head giving nearly 360 degree vision and make out shapes and colors. They have the ability to see above and behind, in front and below, a benefit enabling them to see predators when feeding.

Olfactory - There olfactory organs are used for smell and are located at the base of the nares and are located between the eyes and the mouth. 

Taste - Taste buds are located around the lips, mouth, and tips of the barbels.

Lateral line - Along the middle of each side of Koi, there is a line of scales connected underneath by a small tubal canal. The lateral line is a sensory organ used to sense vibrations in the water. The lateral line runs at approximately the same level as the spine and is an important landmark for locating the blood vessel.

Internal Anatomy 

Bones - Koi have a bony skeleton that supports and protects the internal organs and tissues. Parts of the first four vertebrae form the Weberian ossicles, a series of tiny bones that link the swim bladder to the inner ear. 

Muscles - There is dark muscle and white muscle. White muscle is for fast swimming and dark muscle is for sustained swimming and is located directly under the skin. This is clinically significant as dark muscle is considered vascularized which facilitates the absorption of medication faster than white muscle into the blood.

Gills - Healthy and undamaged gills are very important. They function to bring oxygen to blood vessels through the passage of oxygenated water in and carbon dioxide out and remove waste from the Koi, namely ammonia. 

Circulatory  - The circulatory system transports nutrients, oxygen and waste. The heart pumps the blood to the vessels, first to the gills, then the brain, then the remaining body parts and back to the heart. Oxygen enters through the gills, is pumped through the system and returns removing carbon dioxide through the gills. Nutrients are transported to the tissues and as they process food they produce nitrogen waste products which are poisonous to the tissues. The nitrogen waste and excess water is transported to the kidneys and are excreted and processed into urine. The majority of the nitrogenous waste leaves the Koi's body through the gills in the form of ammonia.

Kidneys - There are two kidneys, a caudal and cranial kidney. 

Liver - The liver has two lobes and is dark red-brown color. The right lobe covers the gallbladder and the left lobes encases the spleen.

Swim bladder - The swim bladder is directly below the kidney and is comprised of two chambers.

Gut - Mechanical break down and mixing of food starts at the pharyngeal teeth; chemical breakdown and absorption occurs in the long intestine. Once the nutrients are absorbed, the remaining solid/undigestable material is voided. Koi do not have a stomach. When Koi have a full intestine during winter the lining may become damaged and allow bad bacteria to enter throgh the lining and get into the blood stream. They may show no sign of this until spring when it warms up.  

Pancreas - The pancreas produces digestive enzymes which are released into the gut to breakdown consumed food.

Gonads -  Males have testes which are white and females have ovaries which are pink, located internally. Some Koi can have both male and female gonadal tissues. Gonads enlarge during breeding season. There is also a vent that is separate from the waste vent. Females can become egg bound requiring attention.

To learn more, take the K.O.I. Anatomy class - it's done entirely on-line, at your own pace.  You read the material and take a short test at the end.  Cost is $25,and members get a 10% diccount!  CLICK HERE to read more about the course or sign up!